High-lift leading edge devices are often used on an aircraft to increase the available maximum lift, particularly during take-off and landing. Two known high-lift leading edge devices are the slat and the drooped leading edge flap.
Slats are generally moveable between a retracted position in which the slat is located against the leading edge of the wing and a deployed position in which the slat is deployed downwards and forwards away from the main portion of the wing. The interaction between the separate flow fields of the slat and the main wing portion respectively is able to reduce the flow velocity induced by the leading edge of the main wing portion and change the pressure gradients on the wing surface to reduce the susceptibility to detachment of airflow from the wing, thereby allowing the aircraft to be flown at higher angles of attack than could otherwise be achieved. The slats are typically mounted for movement on tracks which can be bulky and heavy.
Drooped leading edge flaps, typified by those disclosed in U.S. Pat. No. 4,200,253 and EP 0 302 143, are moveable between a stowed position and a deployed position in which the drooped leading edge flap is turned nose-down. During high incidence flight a drooped leading edge flap reduces the leading edge suction peak experienced by the free stream onset flow. This is due to the deployed leading edge being better aligned to the freestream such that the flow does not negotiate the relatively low radius of curvature of the clean leading edge. The result is a reduction in the acceleration experienced by the flow and a subsequent reduction in the severity of the adverse decelerating pressure gradient further downstream. The net effect is a delay in the onset of wing stall to higher incidences compared to the undeployed wing geometry
It will be understood, therefore, that the benefits of the slat and drooped leading edge flap respectively, exist as a result of different aerodynamic behaviour. In addition, the two devices differ in that movement of a slat is usually described by a combination of a component of rotation and a relatively large component of translation, whereas the movement of a drooped leading edge device is usually described by just a rotation, as shown for example in U.S. Pat. No. 4,200,253 and EP 0 302 143, with little or zero translational movement Also, drooped leading edge flaps may be inherently quieter, and better meet current noise level requirements, than other high-lift devices such as slats.
Drooped leading edge flaps are generally sealed or abutted against the adjacent portion of the main wing on the upper and/or lower surfaces to form a continuous aerofoil surface. If the design of the drooped leading edge flap were such that a discontinuity could be formed between the upper trailing edge of the drooped leading edge flap and the adjacent portion of the main wing (despite there being a seal between), a step might be formed that could disrupt the air flow over the wing, reducing the maximum lift coefficient and increasing the drag. As such, drooped leading edge flaps of the prior art are designed so that in use a substantially continuous surface is maintained between a drooped leading edge flap and the adjacent portion of the wing (the main wing element). As such, the movement of the drooped leading edge flap over the main wing element leading edge, as occurs during deployment, requires a complex sealing system or camming arrangement, such as that shown in EP 0 302 143. Such a system or arrangement may often only be reliably achieved with the addition of considerable weight, complexity and cost.
The present invention seeks to provide an aircraft that is able to benefit from the advantages of a high lift device, such as a drooped leading edge flap or a slat, whilst mitigating at least some of the above-mentioned problems.